Prostaglandin I.sub.2 (PGI.sub.2) is a physiologically active natural substance having the following structural formula, which is biosynthesized from Prostaglandin H.sub.2 (PGH.sub.2) in the metabolic process in vivo called arachidonate cascade. ##STR3## (see Nature, 263, 663(1976), Prostaglandins, 12, 685(1976), ibid, 12, 915(1976), ibid, 13, 375(1977) and Chemical and Engineering News, Dec. 20, 17(1976).
PGI.sub.2 has been confirmed to possess not only a very strong inhibitory activity on blood platelet aggregation but a dissociative activity on blood platelet aggregation, an inhibitory activity on blood platelet adhesion, a vasodilating activity, an inhibitory activity on gastric acid secretion etc. Therefore, it has been considered that PGI.sub.2 is useful for the prevention and/or the treatment for thrombosis, arteriosclerosis, ischemic heart diseases, gastric ulcer, hypertension etc. But its application for pharmaceuticals is limited because of its chemical instability and difficulty of separation of the actions according to purpose. Accordingly, various PGI.sub.2 derivatives have been synthesized and many researches have been carried out for the maintenance and the separation of the actions. However, we have not necessarily satisfactory results yet.
Recently, in order to solve two problems above described, the research for PGI2 receptor agonists which have a new-typed skeleton and may be useful for the treatment of or for the prevention of the above diseases, in view of PGI.sub.2 receptor level, has been carried out.